Remove the default bound from Packable::Visitor #331

Job	Run time
format	14s
build and test (ubuntu-latest, nightly)	1m 54s
missing-files	3s
build and test (ubuntu-latest, beta)	1m 56s
build and test (ubuntu-latest, stable)	1m 50s
build and test (macos-latest, nightly)	1m 55s
build and test (macos-latest, beta)	1m 50s
build and test (macos-latest, stable)	1m 52s
build and test (windows-latest, nightly)	1m 55s
build and test (windows-latest, beta)	1m 54s
build and test (windows-latest, stable)	1m 52s
clippy	1m 4s
udeps	6m 9s
docs	58s
licenses	3m 6s
no_std	0s
miri	0s
	28m 32s

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